1. Technical Field
The present invention relates to a delay-line demodulator and a method of adjusting a phase shift the demodulator.
2. Related Arts
In recent years, studies for realizing an optical transmission system of a high transfer rate (for example, a 40 Gbps transfer rate) have been performed against a backdrop of a rapid spread of a broadband communication. However, in the case of increasing the transfer rate, there is provided a problem that a quality of a communication line deteriorates because a transmit duration per bit of light signals decreases and a signal waveform deteriorates due to a characteristic of an optical fiber. Moreover, it becomes hard to construct a network system with using a conventional fiber network because a repeater is required for such as converting a light signal into an electric signal during passing a transmission path and reconverting the electric signal into a light signal, for performing a long haul transmission of a 40 Gbps transfer rate. Therefore, a differential quadrature phase shift keying (DQPSK) method has been currently investigated and developed, which is able to reduce the deterioration of the signal waveform by widening the transmit duration per bit of a light signal. It becomes able to transmit a distance by using the DQPSK four times longer than that by using a conventional binary phase shift keying (BPSK) method for the 40 Gbps transmission, as the DQPSK is the phase shift keying method to transmit four data (0, 1, 2, 3) with corresponding to four different light phases (θ, θ+π/2, θ+π, θ+3π/2) of a carrier wave. It is considered that it becomes possible to construct the network system among large cities with using the conventional fiber network by such the DQPSK method.
A brief configuration of a conventional optical transmission system (an optical transmitter and receiver) using the DQPSK method is shown in FIG. 4. Such the optical transmission system comprises an optical transmitter 100 and an optical receiver 101. There are provided such as a demodulator 102, balanced receivers 103, 103, an electric circuit 105 and the like in the optical receiver 101.
An optical signal is transmitted from the optical transmitter 100 to an optical fiber 106, as a DQPSK signal including four data (0, 1, 2, 3) modulated into four different light phases (0, π/2, π, 3π/2) of a carrier wave respectively. The DQPSK signal transmitted from the optical fiber 106 to the optical receiver 101 is converted into a light intensity signal in the demodulator 102. Moreover, the light intensity signal is converted into an electric signal by the balanced receiver 103, 103, and the data of the DQPSK signal are demodulated. Furthermore, such as a decoding process or the like is performed in the electric circuit 105.
Such the demodulator 102 is a planar lightwave circuit (PLC) type demodulator as shown in FIG. 5, which is comprised of a Y-branch waveguide 200 and two Mach-Zehnder interferometers (MZIs) 210, 220. Here, a phase of the MZI 220 is required to be shifted as π/2 corresponding to the phase of the MZI 210. Moreover, the MZI 210 comprises: two directional couplers 211 and 212; and two waveguides 213 and 214 having a different waveguide length connected to between the two directional couplers 211 and 212. Meanwhile, the MZI 220 comprises: two directional couplers 221 and 222; and two waveguides 223 and 224 having a different waveguide length connected to between the two directional couplers 221 and 222.
It is quite hard to control precisely a relative phase between the two MZIs 210 and 220 due to a fluctuation of a refractive index of a glass member during fabricating the PLC. Therefore, it is required to adjust the phase of the two MZIs 210 and 220 using a phase trimming technology for adjusting the phase at the parts of the waveguides 213 and 223 after fabricating the PLC. Such the phase trimming technology is developed in a variety of types. Among the various types of the phase trimming technology, a local heat phase trimming technology with using a change in a permanent refractive index by localized heating using a thin film heater is an actual method for realizing the phase trimming in high accuracy without particular devices and equipments being required.
Therefore, thin film heaters 215 and 225 are formed on the waveguides 213 and 223 for phase trimming as shown in FIG. 5. Regarding the local heat phase trimming, it is considered that an equivalent refractive index of a core is changed by a photoelastic effect, because a stress internalized in a PLC chip or a stress caused by the thin film heaters is irreversibly changed by heater heating of a localized and high power (several W/mm). There is disclosed such the local heat phase trimming technology for example in a published Japanese patent application No. 2005-092217 (hereinafter, it is described as a document 1), or there is reported by Kawashima et. al, IEICE Electronics Society of Japan 2006, C-3-12 (hereinafter, it is described as a document 2).
Regarding the above mentioned local heat phase trimming, while a relatively large amount of phase shift is able to be obtained in proportion to a heat duration or a power, there is a problem that a polarization dependent frequency (PDF) becomes gradually large (refer to the document 2). Especially in the demodulator for the optical transmission system (the optical transmitter and receiver) using the DQPSK method, it does not work as a device when the PDF becomes large, due to a narrow spectrum width thereof. For example, an allowable PDF is approximately 0.1 GHz for the optical transmitter and receiver using the DQPSK method of the 40 Gbps. Hence, it is quite hard to realize the demodulator having the small PDF.